My design (4-layer, FR4, 1.6mm) has a couple of ICs with 0.5mm pitch pads and some LEDs with 0.4mm pitch pads, very small LEDs. Should I consider ENIG (more expensive) or will HASL do the job? I read that when doing BGA it is most important to have a board finish with ENIG but what is the pitch limit for QFN that HASL finish is good enough? Is there a difference when using leaded or non-leaded solder?
Thanks.
There are many reasons to choose ENIG over HASL. In fact, very usually I spend a few bucks more on it, even in prototypes (though your fabs may have different pricing models), because it's just that much easier to get the solder contact right on any component, even if there's not small gaps.
Usually your fab can best supply information of when HASL will flood over between islands (i.e. Minimum pin-pin clearance for HASL). The reason for BGA's and such to choose ENIG is the extreme flatness of ENIG compared to HASL. On HASL you don't know for-certain-enough that all balls will sink onto the pads in a close enough time frame, if one side has most balls connecting and one side doesn't it might start to topple or drift. With ENIG that risk is one or more order(s) of magnitude less significant.
Most of my PCB-Only Fabs give 10mil (0.254mm) as lowest lower advisable limit for the HASL.
Most production houses I work with (that do the assembly) kindly request ENIG or similar over HASL from 15mil (0.381mm) pad clearance and downward. Sometimes they will advise ENIG below 20 mil clearance if there's very large TQFPs (>160pin), to avoid skewing risks. But often those size TQFPs are already 15mil or below pin clearance. If you design the connections well with the right thermal reliefs and neckdowns the risk is very small of them running off.
Another reason to choose ENIG regardless of pin clearance or BGA is if your TQFPs have thermal pads, these are also easier to get to flow out neatly on ENIG due to its flatness, but can usually end up well with HASL as well.
Personally, if I had tiny stuff on there (<=0.5mm pad to pad) and I didn't want to get into a fight with a production house (or my tweezers, as the case may be) when there's problems with the finished product, I find ENIG a worthy investment. And if you don't, reserve an area where you can take a nice graphic out of the solder resist (don't do wave soldering, or it'll be a waste ;-)) and that will come out nice and gold for eternity.
